4-amino-1-isobutyl-1H-imidazo[4,5-c]quinoline, also known as imiquimod, is an immune response modifier, indicated for treating genital warts. Imiquimod is also indicated for topical treatment of clinically typical, nonhyperkeratotic, nonhypertrophic actinic keratoses on the face or scalp in immunocompetent adults and for the treatment of biopsy-confirmed, primary superficial basal cell carcinoma in immunocompetent adults, with a maximum tumor diameter of 2.0 cm, located on the trunk (excluding anogenital skin), neck, or extremities (excluding hands and feet).
The drug is marketed as a 5% cream under the trade name Aldara® and has the following structural formula (I):

The synthesis of imiquimod was described in several patents, for example in U.S. Pat. Nos. 4,689,338 and 4,929,624 (to Minnesota Mining and Manufacturing Co. Inc.). The final step of the processes described therein involves an ammonolysis reaction carried out by heating the compound 4-chloro-1-isobutyl-1H-imidazo[4,5-c]quinoline of formula (II) in the presence of ammonium hydroxide or ammonia in methanol under high pressure (e.g. in a steel bomb) at 150° C. to afford imiquimod of formula (I), as depicted in Scheme 1.

U.S. Pat. No. 4,988,815 describes another process for preparing imiquimod, wherein the final step similarly involves ammonolysis of the compound 4-chloro-1-isobutyl-1H-imidazo[4,5-c]quinoline. The ammonolysis reactions described in U.S. Pat. Nos. 4,689,338 and 4,929,624 are disadvantageous in that the reactions require high temperature and high pressure, which is undesirable from the standpoint of industrial safety.
WO 2006/070379 discloses yet another process for preparing imiquimod, wherein the compound 4-chloro-1-isobutyl-1H-imidazo[4,5-c]quinoline is converted to 4-iodo-1-isobutyl-1H-imidazo[4,5-c]quinoline by reacting it with sodium iodide in acetone followed by converting the 4-iodo-1-isobutyl-1H-imidazo[4,5-c]quinoline to imiquimod by using methanolic ammonia at a temperature of 150-155° C. and high pressure of about 20 bars.
US 2007/0010675 (hereinafter the '675 application) discloses a process for purifying imiquimod via its organic acid addition salts, e.g., the imiquimod formate salt. The '675 application states that “the processes used for imiquimod preparation usually do not provide highly pure imiquimod so that, for regulatory requirements to be fulfilled, imiquimod has to be subjected to a subsequent purification process.”
Thus, there is a need in the art for an improved process for preparing highly pure imiquimod from 4-chloro-1-isobutyl-1H-imidazo[4,5-c]quinoline that will be more suitable for industrial use in comparison to conventional processes and will enable the preparation of highly pure imiquimod under more industrially feasible conditions. There is also a need for a simple, straightforward process for obtaining highly pure imiquimod. The present invention provides such a process.